Electrochemical Oscillation during Galvanostatic Charging of LiCrTiO4 in Li-Ion Batteries

In the late 1960s, the establishment of Prigogine’s dissipative structure theory laid the foundation for the (electro)chemical oscillation phenomenon, which has been widely investigated in some electrochemical reactions, such as electro-catalysis and electro-deposition, while the electrochemical oscillation of Li-ion batteries has just been discovered in spinel Li4Ti5O12 a few years before. In this work, spinel LiCrTiO4 samples were synthesized by using a high-temperature solid-state method, characterized with SEM (Scanning electron microscope), XRD (X-ray diffraction), Raman and XPS (X-ray photoelectron spectroscopy) measurements, and electrochemically tested in Li-ion batteries to study the electrochemical oscillation. When sintering in a powder form at a temperature between 800 and 900 °C, we achieved the electrochemical oscillation of spinel LiCrTiO4 during charging, and it is suppressed in the non-stoichiometric LiCrTiO4 samples, especially for reducing the Li content or increasing the Cr content. Therefore, this work developed another two-phase material as the powder-sintered LiCrTiO4 exhibiting the electrochemical oscillation in Li-ion batteries, which would inspire us to explore more two-phase electrode materials in Li-ion batteries, Na-ion batteries, etc.


Introduction
Li-ion batteries have been widely used in portable electronic devices and electric vehicles, [1][2][3] and graphite is a commercial anode material in Li-ion batteries, while it suffers from poor rate capability and serious safety issues, owing to lithium dendritic growth, especially at low temperature [4,5]. Spinel Li 4 Ti 5 O 12 is considered among the promising anode materials, which can prevent the growth of lithium dendrites to achieve safe and reliable high-power Li-ion batteries, owing to the high operating voltage of 1.565 V [6][7][8][9]. Since Li 4 Ti 5 O 12 has smooth discharge and charge plateaus, even subtle voltage changes can be observed on the discharge/charge curves, such as the memory effect and the electrochemical oscillation [10][11][12][13].
In the late 1960s, the establishment of Prigogine's dissipative structure theory laid the foundation for the (electro)chemical oscillation phenomenon, which attracted a lot of attention in some electrochemical reactions, such as electro-synthesis [14,15], electrocatalysis [16,17] and electrodeposition [18,19]. The electrochemical oscillation can not only be adopted to interpret some interesting electrochemical phenomena, but also be developed for some potential applications [20,21]. However, the electrochemical oscillation of Li-ion batteries has just been discovered in spinel Li 4 Ti 5 O 12 a few years before, which presents as a periodic voltage oscillation during the galvanostatic process in Li-ion batteries. Up to now, there is no report about the electrochemical oscillation of any other two-phase materials in Li-ion batteries.

Experimental Section
Spinel LiCrTiO 4 was synthesized by using a high-temperature solid-state method. The raw materials as Li 2 CO 3 (99.9%, Xilong Chemical, Guangzhou, China), Cr 2 O 3 (99.9%, Aladdin, Shanghai, China) and TiO 2 (nano-sized, 99.9%, Aladdin, Shanghai, China) were weighted with an elemental ratio of Li:Cr:Ti as 1:1:1, their mixture was thoroughly ground for 2 h, then the powder was directly sintered in a muffle furnace with air atmosphere at a temperature of 750, 800, 850, 900 and 950°C, respectively, for 16 h, in which the product of 850°C was marked as LiCrTiO 4 -850-powder [26]. In comparison, the ground powder was pressed into a pellet and sintered in a muffle furnace with air atmosphere at a temperature of 850°C for 16 h, and the product was marked as LiCrTiO 4 -850-pellet. By using the same method, another six LiCrTiO 4 samples were synthesized with an elemental ratio of Li:Cr:Ti as 1.05:1:1, 0.95:1:1, 1:1.05:1, 1:0.95:1, 1:1:1.05 and 1:1:0.95, respectively, which were sintered in a powder form at a temperature of 850 • C.
As-prepared LiCrTiO 4 samples were characterized using an X-ray diffractometer (XRD, Bruker D8 advance, Bruker, Karlsruhe, Germany), a Raman spectrometer (Raman, Thermo Fisher DXRxi, Madison, WI, USA) using an argon laser with a wavelength of 532 nm, a scanning electron microscope (SEM, Phenom ProX, Phenom-World BV, Eindhoven, Netherlands), and an X-ray photoelectron spectroscope (XPS, ESCALAB 250xi, Thermo Fisher Scientific, Waltham, MA, USA) with a focused monochromatized Al-Ka radiation (1486.6 eV). Electrochemical measurements were conducted within coin-type cells (CR2025). The working electrode was a composite film (φ = 4 mm) firmly pressed on a carbon paper, which contained 42.5 wt.% active material, 42.5 wt.% acetylene black and 15 wt.% polytetrafluoroethylene (PTFE), the counter electrode of lithium metal was separated from the working electrode with a Celgard 2500 (Celgard, Charlotte, NC, USA) microporous polypropylene film, and the electrolyte was 1 M LiClO 4 /EC + DEC (volume ratio of 1:1). The dried components were assembled in a glovebox filled with Ar gas. The galvanostatic (dis)charging measurements were conducted at a current rate of 0.1 C between 1.2 and 2.0 V in the Hokuto Denko battery test system (HJ1001SD8, Hokuto Denko Corporation, Gifu, Japan) under an operating temperature of 25 • C. Figure 1 shows the SEM images of LiCrTiO 4 sintered in a powder form at a temperature of 750, 800, 850, 900 and 950 • C, respectively. Each sample is an aggregation of sub-microparticles and microparticles, and the particle size becomes large at the high temperature. As shown in Figure 2a and Figure S1, their XRD patterns are consistent with the standard Bragg reflections in JCPDS No. 47-0139, assigned to a spinel phase with space group Fd-3m [27,28]. There are four peaks at 243, 396, 584 and 665 cm −1 in their Raman spectra, as shown in Figure 2b. The peaks at 584 and 665 cm −1 can be attributed to the vibrational modes of Cr-O bonds in CrO 6 octahedra and Ti-O bonds in TiO 6 octahedra. The peak at 396 cm −1 can be assigned to the stretching vibrational mode of Li-O bonds in LiO 4 tetrahedra. Additionally, the peak at 243 cm −1 is ascribed to the vibration of Li-O bonds [29,30]. For the LiCrTiO 4 sintered at 750°C, the peak at 396 cm −1 is higher and the peak at 665 cm −1 is lower than that of high temperatures. Additionally, Materials 2021, 14, 3624 3 of 11 the peak at 243 cm −1 disappears at the high temperature of 900 and 950 • C. Accordingly, the local structure of LiCrTiO 4 is sensitive to the sintering temperature, compared with its crystal structure. bonds in LiO4 tetrahedra. Additionally, the peak at 243 cm −1 is ascribed to the vibration of Li-O bonds [29,30]. For the LiCrTiO4 sintered at 750 ℃, the peak at 396 cm −1 is higher and the peak at 665 cm −1 is lower than that of high temperatures. Additionally, the peak at 243 cm −1 disappears at the high temperature of 900 and 950 °C. Accordingly, the local structure of LiCrTiO4 is sensitive to the sintering temperature, compared with its crystal structure.   bonds in LiO4 tetrahedra. Additionally, the peak at 243 cm −1 is ascribed to the vibration of Li-O bonds [29,30]. For the LiCrTiO4 sintered at 750 ℃, the peak at 396 cm −1 is higher and the peak at 665 cm −1 is lower than that of high temperatures. Additionally, the peak at 243 cm −1 disappears at the high temperature of 900 and 950 °C. Accordingly, the local structure of LiCrTiO4 is sensitive to the sintering temperature, compared with its crystal structure.   The galvanostatic (dis)charging measurements were conducted for these LiCrTiO 4 samples, as shown in Figure 3, where the current rate was 0.1 C and the operating tem-perature was 25 • C. As shown in Figure 3a,b, the charging curve is very smooth for the LiCrTiO 4 sintered at 750 • C. The electrochemical oscillation (voltage oscillation) appears for the LiCrTiO 4 sintered at 800 • C, and its range and amplitude increase continuously as the temperature rises from 800 to 900 • C. However, the electrochemical oscillation disappears completely when the sintering temperature reaches 950 • C. To analyze the oscillation, the relationship between the period and the average voltage was plotted in Figure 3c. At the beginning of electrochemical oscillation, the period gradually increases, while the average voltage is nearly constant. Then, the period and the average voltage gradually become smaller and higher, respectively, and the period basically has a negative logarithmic relationship with the average voltage. By increasing the temperature, the negative logarithmic lines gradually shift to the large period, possibly owing to the fact that the growing particle size prolongs the time of phase transition in each particle, as shown in Figure 1. Therefore, the electrochemical oscillation of LiCrTiO 4 is significantly dependent on the sintering temperature, and the middle temperature of 850 • C was chosen in the following study, for this temperature is very popular to synthesize the spinel LiCrTiO 4 . The galvanostatic (dis)charging measurements were conducted for these LiCrTiO4 samples, as shown in Figure 3, where the current rate was 0.1 C and the operating temperature was 25 °C. As shown in Figure 3a,b, the charging curve is very smooth for the LiCrTiO4 sintered at 750 °C. The electrochemical oscillation (voltage oscillation) appears for the LiCrTiO4 sintered at 800 °C, and its range and amplitude increase continuously as the temperature rises from 800 to 900 °C. However, the electrochemical oscillation disappears completely when the sintering temperature reaches 950 °C. To analyze the oscillation, the relationship between the period and the average voltage was plotted in Figure 3c. At the beginning of electrochemical oscillation, the period gradually increases, while the average voltage is nearly constant. Then, the period and the average voltage gradually become smaller and higher, respectively, and the period basically has a negative logarithmic relationship with the average voltage. By increasing the temperature, the negative logarithmic lines gradually shift to the large period, possibly owing to the fact that the growing particle size prolongs the time of phase transition in each particle, as shown in Figure 1. Therefore, the electrochemical oscillation of LiCrTiO4 is significantly dependent on the sintering temperature, and the middle temperature of 850 °C was chosen in the following study, for this temperature is very popular to synthesize the spinel LiCrTiO4.          Figure 6 shows the galvanostatic discharge/charge curves of the LiCrTiO4 samples with different elemental ratios. Compared with the stoichiometric LiCrTiO4 (Li:Cr:Ti = 1:1:1), the electrochemical oscillation is reduced in range and amplitude for the non-stoichiometric LiCrTiO4 samples. As to each element, the electrochemical oscillation disappears by reducing the Li content (Li:Cr:Ti = 0.95:1:1) or increasing the Cr content (Li:Cr:Ti = 1:1.05:1), while it always exists by varying the Ti content (Li:Cr:Ti = 1:1:1.05 or 1:1:0.95). As shown in Figure 6c, the negative logarithmic line shifts to the high voltage for the LiCrTiO4 of Li:Cr:Ti = 1.05:1:1, and moves to the low voltage for the LiCrTiO4 of Li:Cr:Ti = 1:1:0.95. Thereby, the electrochemical oscillation strongly depends on the elemental ratio, especially for the Li and Cr contents. Additionally, we also synthetized the composite of LiCrTiO4 + Li2Ti3O7, LiCrTiO4 + Li4Ti5O12 and LiCrTiO4 + Li2TiO3 to study the effect of different lithium titanates, as shown in Figure S3 [31]. As shown in Figure 7a,b, the range of electrochemical oscillation gradually becomes smaller as the Li content in lithium titanate increases, consistent with that of the LiCrTiO4 samples (Li:Cr:Ti = 1.05:1:1 vs.1:1:1) in Figure 6, and the amplitude of electrochemical oscillation is largest for the composite of LiCrTiO4 + Li4Ti5O12, indicating a positive effect of spinel Li4Ti5O12. As shown in Figure 7c, the negative logarithmic lines shift to the low voltage for the LiCrTiO4 composites. Thus, the electrochemical oscillation is affected by the Li content in lithium titanates, as well as spinel Li4Ti5O12 phase in the composite. Additionally, we also synthetized the composite of LiCrTiO 4 + Li 2 Ti 3 O 7 , LiCrTiO 4 + Li 4 Ti 5 O 12 and LiCrTiO 4 + Li 2 TiO 3 to study the effect of different lithium titanates, as shown in Figure S3 [31]. As shown in Figure 7a Usually, the raw materials are pressed into a pellet for sintering in the high-temperature solid-state method, so we also synthesized LiCrTiO4-850-pellet sintered in a pellet form at 850 °C. Compared with LiCrTiO4-850-powder, the LiCrTiO4-850-pellet has a similar XRD pattern and Raman spectrum, while there is an additional minor peak at 857 cm −1 which should originate from the stretching vibration of Cr 6+ =O as shown in Figure 8a,b, and the particles evidently become large in the LiCrTiO4-850 pellet, as shown in Figure 8c. The galvanostatic discharge/charge curves are plotted in Figure 8d, in which there is no electrochemical oscillation at the end of charging plateau. Usually, the raw materials are pressed into a pellet for sintering in the high-temperature solid-state method, so we also synthesized LiCrTiO 4 -850-pellet sintered in a pellet form at 850 • C. Compared with LiCrTiO 4 -850-powder, the LiCrTiO 4 -850-pellet has a similar XRD pattern and Raman spectrum, while there is an additional minor peak at 857 cm −1 which should originate from the stretching vibration of Cr 6+ =O as shown in Figure 8a,b, and the particles evidently become large in the LiCrTiO 4 -850 pellet, as shown in Figure 8c. The galvanostatic discharge/charge curves are plotted in Figure 8d, in which there is no electrochemical oscillation at the end of charging plateau.

Results and Discussion
The XPS measurement was adopted to analyze the difference between LiCrTiO 4 -850-powder and LiCrTiO 4 -850-pellet, as shown in Figure 9. The Li 1s peak at ca. 54.7 eV was observed for both samples, as shown in Figure 9a [32]. There are four binding energies: 576.4 eV for Cr 3+ 2p 3/2 , 579.6 eV for Cr 6+ 2p 3/2 , 586.3 eV for Cr 3+ 2p 1/2 and 588.9 eV for Cr 6+ 2p 1/2 , as shown in Figure 9b [33][34][35]. The ratio between Cr 6+ and Cr 3+ is 0.735:1 for LiCrTiO 4 -850-powder and 0.632:1 for the LiCrTiO 4 -850-pellet, according to their peak areas. The Cr element exists in the form of Cr 3+ in spinel LiCrTiO 4 , and the Cr 6+ ions on the surface should be attributed to the high-temperature sintering in air atmosphere, especially for LiCrTiO 4 -850-powder. As shown in Figure 9c, the Ti 2p 3/2 peak at 458.4 eV and Ti 2p 1/2 peak at 464.1 eV indicate that the Ti element mainly exists in the form of Ti 4+ [36]. As for the O 1s spectra in Figure 9d, the binding energies of 529.8 eV and 531.2 eV are assigned to the lattice oxygen and the CO 3 2− , respectively [37][38][39]. Here, the raw material of Li 2 CO 3 might leave the CO 3 2− on the particle surface, especially for LiCrTiO 4 -850-pellet. Therefore, the LiCrTiO 4 -850-powder has more Cr 6+ and less CO 3 2− on the particle surface than the LiCrTiO 4 -850-pellet, which should be correlative to the occurrence of electrochemical oscillation. The XPS measurement was adopted to analyze the difference between LiC-rTiO4-850-powder and LiCrTiO4-850-pellet, as shown in Figure 9. The Li 1s peak at ca. 54.7 eV was observed for both samples, as shown in Figure 9a [32]. There are four binding energies: 576.4 eV for Cr 3+ 2p3/2, 579.6 eV for Cr 6+ 2p3/2, 586.3 eV for Cr 3+ 2p1/2 and 588.9 eV for Cr 6+ 2p1/2, as shown in Figure 9b [33][34][35]. The ratio between Cr 6+ and Cr 3+ is 0.735:1 for LiCrTiO4-850-powder and 0.632:1 for the LiCrTiO4-850-pellet, according to their peak areas. The Cr element exists in the form of Cr 3+ in spinel LiCrTiO4, and the Cr 6+ ions on the surface should be attributed to the high-temperature sintering in air atmosphere, especially for LiCrTiO4-850-powder. As shown in Figure 9c, the Ti 2p3/2 peak at 458.4 eV and Ti 2p1/2 peak at 464.1 eV indicate that the Ti element mainly exists in the form of Ti 4+ [36]. As for the O 1s spectra in Figure 9d, the binding energies of 529.8 eV and 531.2 eV are assigned to the lattice oxygen and the CO3 2− , respectively [37][38][39]. Here, the raw material of Li2CO3 might leave the CO3 2− on the particle surface, especially for LiCrTiO4-850-pellet. Therefore, the LiCrTiO4-850-powder has more Cr 6+ and less CO3 2− on the particle surface than the LiCrTiO4-850-pellet, which should be correlative to the occurrence of electrochemical oscillation.   The XPS measurement was adopted to analyze the difference between LiC-rTiO4-850-powder and LiCrTiO4-850-pellet, as shown in Figure 9. The Li 1s peak at ca. 54.7 eV was observed for both samples, as shown in Figure 9a [32]. There are four binding energies: 576.4 eV for Cr 3+ 2p3/2, 579.6 eV for Cr 6+ 2p3/2, 586.3 eV for Cr 3+ 2p1/2 and 588.9 eV for Cr 6+ 2p1/2, as shown in Figure 9b [33][34][35]. The ratio between Cr 6+ and Cr 3+ is 0.735:1 for LiCrTiO4-850-powder and 0.632:1 for the LiCrTiO4-850-pellet, according to their peak areas. The Cr element exists in the form of Cr 3+ in spinel LiCrTiO4, and the Cr 6+ ions on the surface should be attributed to the high-temperature sintering in air atmosphere, especially for LiCrTiO4-850-powder. As shown in Figure 9c, the Ti 2p3/2 peak at 458.4 eV and Ti 2p1/2 peak at 464.1 eV indicate that the Ti element mainly exists in the form of Ti 4+ [36]. As for the O 1s spectra in Figure 9d, the binding energies of 529.8 eV and 531.2 eV are assigned to the lattice oxygen and the CO3 2− , respectively [37][38][39]. Here, the raw material of Li2CO3 might leave the CO3 2− on the particle surface, especially for LiCrTiO4-850-pellet. Therefore, the LiCrTiO4-850-powder has more Cr 6+ and less CO3 2− on the particle surface than the LiCrTiO4-850-pellet, which should be correlative to the occurrence of electrochemical oscillation.   Figure 10 shows the research progress in the electrochemical oscillation of Li-ion batteries, which was first discovered during the galvanostatic charge and discharge processes of spinel Li 4 Ti 5 O 12 prepared by carbothermic method, as shown in Figure 10a. Here, our work reveals the electrochemical oscillation during the galvanostatic charge process of spinel LiCrTiO 4 , as shown in Figure 10b. Olivine LiFePO 4 is a well-known two-phase cathode material in Li-ion batteries, but no electrochemical oscillation has been observed during the galvanostatic discharge and charge processes, as shown in Figure 10c, possibly owing to the anisotropic crystal structure and the poor electrical conductivity of LiFePO 4 . Thereby, the research of electrochemical oscillation has been expanded to the charge process of spinel LiCrTiO 4 , inspiring us to explore more two-phase electrode materials in Li-ion batteries, Na-ion batteries, etc., as shown in Figure 10d. two-phase cathode material in Li-ion batteries, but no electrochemical oscillation has been observed during the galvanostatic discharge and charge processes, as shown in Figure 10c, possibly owing to the anisotropic crystal structure and the poor electrical conductivity of LiFePO4. Thereby, the research of electrochemical oscillation has been expanded to the charge process of spinel LiCrTiO4, inspiring us to explore more two-phase electrode materials in Li-ion batteries, Na-ion batteries, etc., as shown in

Conclusions
In this work, spinel LiCrTiO4 samples were synthesized by using a high-temperature solid-state method, characterized with SEM, XRD, Raman and XPS measurements, and electrochemically tested in Li-ion batteries to study the electrochemical oscillation. Through tailoring the sintering temperature, we found that the electrochemical oscillation during charging can be observed for the temperatures 800 to 900 °C, of which the range and amplitude increase with the temperature. Compared with the stoichiometric LiCrTiO4, the electrochemical oscillation is reduced in range and amplitude for the non-stoichiometric LiCrTiO4 samples, and it even disappears by reducing the Li content or increasing the Cr content. When the LiCrTiO4 is sintered in a pellet form, there is no electrochemical oscillation in the end of charging plateau. According to the XPS results, the powder-sintered LiCrTiO4 has more Cr 6+ and less CO3 2− on the particle surface than the pellet-sintered LiCrTiO4, which should be correlative to the occurrence of electrochemical oscillation. Thereby, this work developed another two-phase material as the powder-sintered LiCrTiO4 to study the electrochemical oscillation, which would encourage us to explore more two-phase electrode materials in Li-ion batteries, Na-ion batteries, etc.

Conclusions
In this work, spinel LiCrTiO 4 samples were synthesized by using a high-temperature solid-state method, characterized with SEM, XRD, Raman and XPS measurements, and electrochemically tested in Li-ion batteries to study the electrochemical oscillation. Through tailoring the sintering temperature, we found that the electrochemical oscillation during charging can be observed for the temperatures 800 to 900 • C, of which the range and amplitude increase with the temperature. Compared with the stoichiometric LiCrTiO 4 , the electrochemical oscillation is reduced in range and amplitude for the non-stoichiometric LiCrTiO 4 samples, and it even disappears by reducing the Li content or increasing the Cr content. When the LiCrTiO 4 is sintered in a pellet form, there is no electrochemical oscillation in the end of charging plateau. According to the XPS results, the powder-sintered LiCrTiO 4 has more Cr 6+ and less CO 3 2− on the particle surface than the pellet-sintered LiCrTiO 4 , which should be correlative to the occurrence of electrochemical oscillation. Thereby, this work developed another two-phase material as the powder-sintered LiCrTiO 4 to study the electrochemical oscillation, which would encourage us to explore more twophase electrode materials in Li-ion batteries, Na-ion batteries, etc.